1. Filed of the Invention
The present invention relates to improvements in an electric double layer capacitor, particularly in activated carbon which forms activated carbon electrodes included in polarizable electrodes.
2. Description of the Background Art
Generally, an electric double layer capacitor has an electric double layer formed at the surface of separation or interface between activated carbon electrodes and electrolyte included in polarizable electrodes, and electric charge is stored in said electric double layer. The activated carbon used for activated carbon electrodes is produced by carbonizing and activating vegetable materials (wood, wood flour, coconut shell, lining, pulp waste water, etc.), mineral materials (coal, coke, pitch, etc.) or synthetic resins (phenol, polyacrylonitrile, cellulose, vinyl chloride, etc.).
In conventional electric double layer capacitors, there exist on the surface of the activated carbon such oxygen functional groups as carboxyl group, carbonyl group and phenolic hydroxy group. If these functional groups exist in large quantities, the affinity of the activated carbon electrodes for the electrolyte improves and the area of the interface between the activated carbon electrodes and the electrolyte increases, so that the electrostatic capacity is increased and stabilized.
However, these functional groups decompose when an electrical potential is imposed on the electrode; therefore, if they exist in large quantities, they deteriorate the voltage retaining characteristic of the capacitor. Therefore, the idea of increasing the electrostatic capacity by increasing oxygen functional groups could not be adopted where the general characteristics of an electric double layer capacitor must be taken into consideration.